Differential pressure sensor device

ABSTRACT

A MEMS differential pressure sensing element is provided by two separate silicon dies attached to opposite sides of a silicon or glass spacer. The spacer is hollow. If the spacer is silicon, the dies are preferably attached to the hollow spacer using silicon-to-silicon bonding provided in part by silicon oxide layers. If the spacer is glass, the dies can be attached to the hollow spacer using anodic bonding. Conductive vias extend through the layers and provide electrical connections between Wheatstone bridge circuits formed from piezoresistors in the silicon dies.

BACKGROUND

Many silicon-based micro-sensors use so-called MEMS(microelectromechanical systems) technology to achieve low cost and highperformance. One such a device is a MEMS pressure sensor, which iscomprised of a small, thin silicon diaphragm onto which a piezoresistivecircuit, normally a Wheatstone bridge, is formed. Diaphragm stressescaused by pressure applied to the diaphragm change the resistance valuesof the piezoresistors in the bridge circuit. An electronic circuitdetects the resistance changes of the piezoresistive bridge and outputsan electrical signal representative of the applied pressure.

FIG. 1A is a cross-sectional view of a prior art differential pressuresensor 100, so named because it provides an output signal representativeof the pressure difference between the top pressure and the bottompressure on the diaphragm 122 of FIG. 1B of the differential pressuresensing element 110 shown in FIG. 1B. FIG. 1B shows a cross-sectionaldiagram of a differential pressure sensing element mounted inside thehousing depicted in FIG. 1A.

In FIG. 1A, the pressure sensor 100 is comprised of a housing 104 thatencloses a MEMS pressure sensing element 110 and an application-specificintegrated circuit (ASIC) 106. One fluid pressure from liquids or gasesis applied to the bottom of the diaphragm of the MEMS pressure sensingelement through a pressure port 108 formed into the housing 104. Theother fluid pressure from gases through the cover 107 is applied to thetop of the gel 124 which passes the pressure to the top of the diaphragmof the MEMS pressure silicon sensing element (or silicon die). The MEMSpressure sensing element 110 is electrically connected to ASIC 106 byconductive wires 103, well-known in the prior art and which provideelectrical connections between the ASIC 106 and the pressure sensingelement 110. Conductive wires also connect the ASIC 106 to theleadframes 105 for the input and output voltages.

As stated above, FIG. 1B is a cross-sectional diagram of a prior artMEMS pressure sensing element packaging 102 comprised of a thin silicondie 110 for differential pressure sensing. A piezoresistive Wheatstonebridge circuit 112 is formed in the die 110 and located near the edge ofa thin diaphragm region 114.

The die 110 sits atop a pedestal 116, which is in turn attached to thebottom 118 of the housing 104 by an adhesive 120. Fluid that flows inthe port 108 applies pressure to the bottom of diaphragm 122 formed bythe placement of the die 110 over the port 108. The other fluid flows tothe top of gel 124 and pressurizes the top of diaphragm 122. Arrow 123represents pressure applied to the top of the diaphragm and arrow 133represents pressure applied to the bottom of the diaphragm. A differenceor differential between the pressure 123 applied downwardly and thepressure 133 applied upwardly causes the diaphragm 122 to deflect. Thedeflection caused by the pressure difference causes the piezoresistorsin the bridge circuit 112 to change their physical dimensions which inturn changes their resistive values. The MEMS pressure sensing element110 shown in FIG. 1B can be seen in FIG. 1A embedded in a conventionalgel 124, an intended function of which is to protect the sensing element110.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a prior art differential pressuresensor;

FIG. 1B is a cross-sectional view of a prior art MEMS sensing elementpackaged in the pressure sensor shown in FIG. 1A;

FIG. 2 is a cross-section of a differential pressure sensing element;

FIG. 3 shows how two Wheatstone bridge circuits are connected in a firstembodiment;

FIG. 4A is a view of the second side of the first silicon die and showsbond pads to which electrical connections are made;

FIG. 4B shows the first side of a first silicon die and connections to afirst Wheatstone bridge circuit thereon;

FIG. 4C shows the bond pad on the oxide layer between the first silicondie and the hollow spacer;

FIG. 4D shows the vias through the hollow spacer;

FIG. 4E shows the bond pads on the oxide layer between the hollow spacerand the second silicon die;

FIG. 4F shows the first side of a second silicon die and connections toa second Wheatstone bridge circuit thereon;

FIG. 5 shows how two Wheatstone bridge circuits are connected in asecond embodiment;

FIG. 6A is a view of the second side of the first silicon die and showsbond pads to which electrical connections are made;

FIG. 6B shows the first side of a first silicon die and connections to afirst Wheatstone bridge circuit thereon;

FIG. 6C shows the bond pad on the oxide layer between the first silicondie and the hollow spacer;

FIG. 6D shows the vias through the hollow spacer;

FIG. 6E shows the bond pads on the oxide layer between the hollow spacerand the second silicon die;

FIG. 6F shows the first side of a second silicon die and connections toa second Wheatstone bridge circuit thereon;

FIG. 7 is a cross-sectional view of a pressure sensor embodiment usingthe pressure sensing element depicted in FIG. 2 with Embodiment 1 asshown in FIGS. 4A-4F or FIG. 2 with Embodiment 2 as shown in FIGS.6A-6F; and

FIG. 8 is a cross sectional view of an alternate embodiment of apressure sensor.

DETAILED DESCRIPTION

The gel 124 used in MEMS sensors tends to be bulky and massive. It cantherefore adversely affect a MEMS pressure sensor responsiveness deviceduring vibration.

Electrical charges in a gel 124 can also tend to distort the electricalproperties of the piezoresistors from which the Wheatstone bridgecircuits are formed. A differential pressure sensor that can eliminatethe need for gel would be an improvement over the prior art.

FIG. 2 is a cross-sectional diagram of a differential pressure sensingelement 200 preferred embodiment. The pressure sensing element 200 iscomprised of a spacer 202 having a top side 204 and a bottom side 206. Acentrally-located hole 208 formed into the spacer 202 extends throughthe top side 204 and the bottom side 206. The hole 208 thus makes thespacer 202 hollow.

The hollow spacer 202 is preferably made of either amorphous orcrystalline silicon but can be also made of borosilicate glass. Silicondies can be attached to the silicon spacer 202 using silicon-to-siliconbonding. Silicon dies can be attached to a borosilicate glass spacer 202by anodic bonding.

In FIG. 2, the top side 204 of the hollow spacer 202 is overlaid with afirst silicon die 210 having a thickness between about 5 and about 100microns. The first silicon die 210 can also be considered a “top” die inthat it is attached to the top side 204 of the spacer 202. A first side212 of the top die 210 faces downwardly, i.e., toward the top side 204of the hollow spacer 202. An opposite second side 214 of the die 210faces upwardly, i.e., away from the top side 204 of the spacer 202 andis formed to have a recess or cavity 229. The die 210 covers the topside 204 of the hollow spacer 202, including the hole 208, which extendsthrough the spacer 202.

A first piezoresistive Wheatstone bridge circuit 216 is formed in thefirst side 212 of the die 210 near the side wall 211 of the cavity 229of the first silicon die 210. The first Wheatstone bridge circuit 216 istherefore located near the edge of the diaphragm 213 formed by the firstsilicon die 210. The first silicon die 210 is attached to the top side204 of a hollow silicon spacer 202 by silicon-to-silicon bondingprovided by a silicon oxide layer 219.

A second silicon die 224 is attached to the bottom side 206 of thehollow spacer 202 by a second oxide layer 220. Since the second silicondie 224 is attached to the bottom side 206 of the spacer 202, it is alsoreferred to as a bottom die 224. It also covers the hole 208 formed intothe spacer 202.

The second silicon die 224 is not necessarily thicker than the firstsilicon die 210. The thicker die is attached to the port for bettersensor packaging to isolate the thermal stress from the substrate. Asshown in FIG. 7 with wire bonding, the thicker die is the second silicondie 224. Nevertheless in FIG. 8 with flip-chip bonding, the thicker dieis the first silicon die 210. The second silicon die 224 has a top orfirst side 226 and a bottom or second side 228. The top or first side226 of the faces upwardly, i.e., toward the bottom 206 of the hollowspacer 202. The second side 228 of the second silicon die 224 facesdownwardly and is processed to have a pressure cavity 230 that extendsupwardly from the bottom 228 of the second die 224 toward the top side226. The pressure cavity 230 stops near the first side 226 of the seconddie 224 to define a thin membrane or diaphragm 232. The thickness of thediaphragm 232 in the second die 224 is between about 5 and about 100microns.

A second Wheatstone bridge circuit 234 is formed into the first side 226of the second silicon die 224. The second Wheatstone bridge circuit islocated in the first side 226 of the second silicon die 224 near andabove the edge 217 of the pressure cavity 230, which is formed from thesecond side 228 of the second die 224 toward the first side 226 of thesecond die 224. The first die 210 and the second die 224 cover the hole208 of the spacer 202 to form a vacuum cavity for the top and bottomabsolute pressure sensing. The diaphragm 232 of the second die 224deflects upwardly and downwardly responsive to pressure applied to thesecond die 224.

As with the first Wheatstone bridge circuit 216, the second Wheatstonebridge circuit 234 is formed near the sidewall 217 of the cavity 230.The second Wheatstone bridge 234 is therefore near the edge 217 of thediaphragm 232 comprised of the second die 224.

The two Wheatstone bridge circuits 216 and 234 are formed frompiezoresistors deposited into the dies using known techniques. Thevalues of the resistors change in response to the deformation of eachrespective diaphragm of the silicon dies. When a voltage is input to theWheatstone bridge circuits, their output voltages change in response todeflection of the dies or the differential pressure on the dies.Electrical connections to the Wheatstone bridge circuits' inputs andoutputs are provided by conductive layers formed into the respectivedies.

When the hole 208 is covered by the first and second dies, both of whichare bonded to the top and bottom sides of the spacer 202, the hole 208becomes a hermetically sealed cavity 208. By sealing the hole 208 whenit is at least partially evacuated, as will happen when the dies andspacer are assembled in an evacuated chamber, a negative pressure orvacuum is maintained inside the evacuated cavity formed by the evacuatedhole 208 and the two dies that cover and seal it. When differentpressures are applied against the two dies, their vertical deflections,relative to each other will differ causing the resistive values ofpiezoresistors within the dies to change in value.

FIG. 3 shows a first topology of two Wheatstone bridge circuits 300 and302 formed using four, “R”-valued piezoresistors formed into the silicondies using processes known in the prior art. Resistors R1 and R2 areconnected in series to each other. Resistors R3 and R4 are connected inseries to each other. The series-connected R1 and R2 are connected inparallel to the series-connected R3 and R4.

The Wheatstone bridge circuits 300 and 302 have two input nodesdenominated as V_(p) and V_(n) and two output nodes denominated as S_(p)and S_(n). V_(p) is usually a small positive voltage, typically aboutthree volts. V_(n) is usually ground or zero volts but V_(n) could alsobe a negative voltage. The nodes between the R-valued piezoresistors inthe first silicon die 210 are provided with electrical interconnects 248formed by P+ conductive silicon interconnects that are deposited ontothe first side 212 of the first silicon die 210. The nodes between theR-valued piezoresistors in the second silicon die 224 are provided withelectrical interconnects 252 formed by P+ conductive siliconinterconnects that are deposited onto the first side 226 of the secondsilicon die 224. The node between R1 and R4 is considered to be thefirst input node V_(p) and is denominated node 1; the node between R2and R3 is considered to be the second input node V_(n) and isdenominated node 2 The node between R3 and R4 is the first output nodeS_(p) and is denominated node 3. The node between R1 and R2 is thesecond output node S_(n) and is denominated node 4.

FIG. 3 shows how the two Wheatstone bridge circuits 300 and 302 areconnected in a first embodiment of a differential pressure sensingelement 200. As shown in FIG. 3, the two Wheatstone bridge circuits areindependent from each other. A direct current voltage is connected toV_(p) and V_(n), i.e., to nodes 1 and 2 respectively The output voltageof the first circuit 300 is taken from S_(p) and S_(n), which are nodes3 and 4 respectively The voltage difference between the voltages atS_(p) and S_(n) is the output voltage.

When a voltage is input to the input terminals V_(p) and V_(n), whichare nodes 1 and 2, the output voltage at the output terminals S_(p) andS_(n) changes in response to changes in the values of thepiezoresistors. Since the piezoresistors are formed into the diaphragms213 and 232 of the thin silicon dies 210 and 224, the nominal resistanceof R ohms will change when the diaphragms deflect in response topressures applied to the diaphragms. In a first embodiment of thepressure sensing element 200, the voltage difference Vdiff between theoutput voltage V1 (V1=Sp1−Sn1) from the first bridge circuit 300 and theoutput voltage V2 (V2=Sp2−Sn2) from the second bridge circuit 302represents a pressure difference, i.e., the difference in pressureapplied to the first silicon die 210 and the pressure applied to thesecond silicon die 224.

Electrical connections to the R-valued resistors that form the firstWheatstone bridge circuit 216 are provided by P+ conductive siliconinterconnects 248 formed into the first side 212 of the die 210. The P+conductive silicon interconnects 248 extend from the R-valuedpiezoresistors over to conductive vias 242 located near the edge of thedie 210 and which extend through the die 210 from the first side 212 toits second side 214. The vias 242 that extend through the die 210terminate at conductive bond pads 244 on the second side 214 of the die210.

As described more fully below with regard to FIG. 7, wires are connectedto the bond pads 244 that extend to an ASIC, best seen in FIG. 7.

Electrical connections of the R-valued piezoresistors of the secondWheatstone bridge circuit 234 of the second silicon die 224 are alsoprovided by way of P+ conductive silicon interconnect 252 formed on thetop side 226 of the second silicon die 224. As with the first die 210,P+ conductive silicon interconnects 252 extend from the R-valuedpiezoresistors of the second Wheatstone bridge circuit 234 over toconductive vias 242 located near the edge of the second die 224 butwhich extend downwardly through the second oxide layer 220, from thehollow spacer 202. The vias 242 thus extend from the second set of P+interconnects 252 on the first side 226 of the second die 224, upwardlythrough the second oxide layer 220, through the hollow spacer 202,through the first oxide layer 219, through the first silicon die 210 tothe aforementioned bond pads 244 on the second side 214 of the first die210.

Since FIG. 2 is a cross-section of the pressure sensing element 200,only one conductive via 242 is shown in the figure. Additional vias 242not visible in FIG. 2 exist in the hollow spacer 202, the oxide layers219 and 220, the first die 210 and the second die 224, which are infront of and behind the via 242 that is visible in FIG. 2. The vias 242,which are comprised of a conductive material formed into holes throughthe various layers, simply act as vertically-oriented conductors ofelectrical signals through the various layers of the pressure sensingelement 200.

The conductive vias 242 are formed by etching holes in the hollow spacer202, the first silicon die 210 and the second silicon die 224 atlocations on each component, which are coincident with each other whenthe hollow spacer 202, oxide layers 219 and 220 and the dies 210 and 224are assembled together as described above. The holes through the layersare filled with a conductive material, examples which include a metal ora doped silicon.

Additional understanding of the structure of the pressure sensingelement 200 depicted in FIG. 2 can be had by other figures that depictthe various layers shown in cross-section in FIG. 2.

FIG. 4A is a top view of the pressure sensing element 200 looking“downwardly” at the second side 214 of the first silicon die 210. Thesecond side 214 of the first silicon die 210 faces away from the hollowspacer 202.

Six square or rectangular bond pads 244 are identified by referencenumerals 244-1 to 244-6. The bond pads 244-1 to 244-6 are effectively ontop of an in electrical connection with conductive vias 242 that extenddownwardly, i.e., into the plane of FIG. 4A through the layers of thedie 210 described above. The bond pads 244-4 and 244-1 are electricalcontacts for the V_(p) and V_(n) power supply voltages that are providedto both Wheatstone bridge circuits 216 and 234. Bond pads 244-2 and244-5 are electrical contacts for the output nodes S_(p) and S_(n) forthe top or first Wheatstone bridge circuit 216, the electrical schematicof which is shown in FIG. 3 and identified by reference numeral 300.Bond pads 244-3 and 244-6 are electrical contacts for the output nodesS_(p) and S_(n) for the bottom or second Wheatstone bridge circuit 234,the electrical schematic of which is shown in FIG. 3 and identified byreference numeral 302. The above layout is only to demonstrate one offunctional designs. The layout of bond pads, conductive interconnects,and vias can be designed in many other different ways.

In FIG. 4A, reference numeral 209 points to a square drawn using abroken line. The square 209 depicts the “footprint” of the sidewall ofthe hole 208 in the hollow spacer 202, which lies below the die 210. Thesquare 229 shows the sidewalls of the cavity on the second side 214 ofthe first silicon die 210.

FIG. 4B is the first side 212 of the first silicon die 210. Statedanother way, FIG. 4B and FIG. 4A are opposite sides of the first silicondie 210.

In FIG. 4B, the first Wheatstone bridge circuit 216 is comprised offour, R-valued P−silicon piezoresistors electrically connected to eachother as shown in the first Wheatstone bridge circuit 300 in FIG. 3.Reference numeral 209 identifies the side wall of the hole 208 in thehollow spacer 202, and which is covered by the first silicon die 210.

As shown in FIG. 3, the Wheatstone bridge circuits have input nodesdenominated as V_(p) and V_(n). The output nodes of the bridge circuitsare denominated as S_(p) and S_(n). In FIG. 4B, the positive andnegative supply voltages, V_(p) and V_(n) for the bridge circuit 216 areavailable at the left-hand side of the die 210 because of theaforementioned conductive vias 242 that extend through the die 210. InFIG. 4B, the two conductive vias identified for consistency purposes byreference numerals 242-4 and 242-1 are connected to the V_(p) and V_(n)input nodes of the Wheatstone bridge 216 via conductive traces 248formed from P+ conductive silicon interconnects deposited onto the firstsurface 212 of the first die 210. In the figure, the output nodes S_(p)and S_(n) of the first Wheatstone bridge 216 are connected to two otherconductive vias, which for consistency purposes are identified in thefigure by reference numerals 242-2 and 242-5.

In FIG. 4B, reference numerals 242-3 and 242-6 “point” to two circles,which are top views of two conductive vias that extend through the firstdie 210 but which extend electrical connections downwardly to lowerlayers of the pressure sensing element 200. The vias 242-3 and 242-6carry signals from the Sp and Sn output nodes of the “bottom” or secondWheatstone bridge circuit 234, which is located in the second die 224.

FIG. 4C is the first silicon oxide spacer or layer 219, which is locatedbetween the first silicon die 210 and the hollow spacer 202. The oxidelayer 219 provides a silicon-to-silicon bond between those twostructures. Six squares or rectangles located at the left-hand side ofFIG. 4C on the oxide layer 219 between the hollow spacer 202 and thefirst die 210 are metal bond pads that are identified by referencenumerals 244-1 to 244-6 to carry the electrical signals V_(n), S_(p) forthe first die, S_(p) for the second die, V_(p), S_(n) for the first die,and S_(n) for the second die, respectively. Reference numerals 242-1through 242-6 identify the vias that extend through the hollow spacer202 and through the first die 210. Reference numeral 209 identifies thesidewall of the hole 208 through the spacer 202, which the oxide layer219 is overlaid.

FIG. 4D is a top or first side 204 of the hollow spacer 202. Fourconductive vias 242-1, 242-3, 242-4, and 242-6 extend through the hollowspacer 202 downwardly, i.e., into the plane of FIG. 4D, and arerepresented by four circles at the left-hand side of the figure. Asshown in FIG. 2, which is a cross section of the pressure sensingelement 200, the vias 242 extend “vertically” through the hollow spacer202 down to the second die 224, which is attached to the lower or secondside 206 of the hollow spacer 202, which is where the lower or secondWheatstone bridge circuit 234 is located in the pressure sensing element200. In FIG. 4D, reference numeral 209 represents the sidewall of thehole 208 through the spacer 202. The vias 242-1, 242-3, 242-4, and 242-6carry signals through the hollow spacer 202 to and from the secondWheatstone bridge circuit 234 on the second silicon die 224.

FIG. 4E is the lower or second silicon oxide layer 220. As shown in FIG.2, it is located between the bottom of the second side 206 of the hollowspacer 202 and the top or first side 226 of the second silicon die 224.Reference numeral 209 identifies the sidewall of the hole 208 formedinto the hollow spacer 202 and around which the oxide layer 220 isattached.

In FIG. 4E, the four small squares on the left-hand side of the figureidentified by reference numerals 244-1, 244-3, 244-4 and 244-6 identifyelectrically conductive metal bond pads surrounding conductive vias242-1, 242-3, 242-4 and 242-6 that carry signals V_(n), S_(p), V_(p) andS_(n) for the second Wheatstone bridge 234 of the second die 224respectively.

FIG. 4F depicts the top or first side 226 of the second silicon die 224,which is located between the second oxide layer 220 and the lower orsecond side 228 of the second silicon die 224. Reference numeral 209identifies the sidewall of the hole 208 formed into the hollow spacer202. The piezoresistors that comprise the second Wheatstone bridgecircuit 234 are electrically connected by P+ interconnects 252 to four,electrically conductive P+ areas 252, which surround bond padsidentified by reference numerals 244-1, 244-3, 244-4 and 244-6. The P+conductive squares 244-1, 244-3, 244-4 and 244-6 are electricallyconnected to the vias for V_(n), S_(p), V_(p) and S_(n) for the secondWheatstone bridge 234 respectively Those vias are identified byreference numerals 242-1, 242-3, 242-4 and 242-6.

FIGS. 4A-4F depict the layers of a first embodiment of a differentialpressure sensor shown in cross section in FIG. 2. Six bond pads 244-1through 244-6 on the top or second side 214 of the first silicon die 210are required to electrically connect the Wheatstone bridge circuits. Twoof the six bond pads are required to connect a power supply to each ofthe two input nodes V_(p) and V_(n) of the two Wheatstone bridgecircuits. The other four bond pads are required for electricalconnections to the S_(p) and S_(n) output nodes of the Wheatstone bridgecircuits. In an alternate embodiment, the number of bond pads is reducedfrom six to four by interconnecting two nodes of the two Wheatstonebridge circuits, within the sensing element layers. FIG. 5 is aschematic diagram of the interconnection of two Wheatstone bridgecircuits by which the two different output voltages of the two circuitscan be determined directly from the circuits themselves. Stated anotherway, in FIG. 5, the output voltage V_(diff) which requires only two bondpads, is the algebraic difference between the output voltage of thefirst Wheatstone bridge 300 and the second Wheatstone bridge circuit302.

FIG. 6A is a view of the second side 214 of the first silicon die 210used in the aforementioned alternate embodiment of the pressure sensingelement 200. Four bond pads 245-1, 245-2, 245-3 and 245-4 areelectrically connected to conductive vias, which are identified in FIGS.6A and 6B by reference numerals 243-1 to 243-4. The conductive vias243-1 to 243-4 extend downwardly from the bond pads 245-1, 245-2, 245-3and 245-4, into the plane of the figure. The conductive vias provideelectrical connections to Wheatstone bridge circuits in the two dies. Bycross connecting Sp and Sn of the two Wheatstone bridge circuits 300 and302 as shown in FIG. 5, only four bond pads, i.e., bond pads 245-1through 245-4, and four vias 243-1 through 243-4 are required to provideall of the connections between the Wheatstone bridge circuits andexternal circuits that are necessary to measure changes in the values ofthe piezoresistors.

In FIG. 5, the V₁ output node is electrically connected to the S_(n)node of the second Wheatstone bridge circuit 302 and to the S_(p) nodeof the first Wheatstone bridge circuit 300. In FIG. 6A, referencenumeral 245-2 identifies a bond pad that is labeled as both S_(n) of thefirst die and S_(p) of the second die.

In FIG. 5, the V₂ output node is electrically connected to the S_(P)node of the second Wheatstone bridge circuit 302 and to the S_(n) nodeof the first Wheatstone bridge circuit 300. In FIG. 6A, referencenumeral 245-4 identifies a bond pad connected to S_(p) of the firstsilicon die 210 and S_(n) of the second silicon die 224. As shown inFIG. 6B, P+ interconnects 248 formed on the first side 212 of the firstsilicon die 210 provide the necessary electrical connections between theS_(p) and S_(n) nodes of the two circuits to reduce the number of bondpads from six to four.

FIG. 6B depicts the layout of P+ interconnects 248 on the first side 212of the first die 210. Reference numeral 209 identifies the sidewall ofthe hole 208 formed into the first side 204 of the hollow spacer 202.The hole 208 lies below the die 210. Reference numerals 243-1, 243-2,243-3 and 243-4 identify conductive vias through the die 210. The P+interconnects 248 electrically connect the vias to the piezoresistors ofthe first Wheatstone bridge 216.

FIG. 6C depicts the layout of the first oxide layer 219 used in thesecond embodiment of the pressure sensing element 200. The first oxidelayer 219 is located between the top of the first surface 204 of thehollow spacer 202 and the first side 212 of the first silicon die 210.Reference numerals 245-1 through 245-4 identify four square orrectangular metal bond pads that make electrical contact with conductivevias 243-1 through 243-4 that extend through the hollow spacer 202 andwhich make electrical contact with the vias that extend through thefirst silicon die 210.

FIG. 6D shows the first side 204 of the hollow spacer 202 that is usedwith the alternate embodiment of the pressure sensing element 200. Fourconductive vias on the left-hand side of the figure are labeled 243-1through 243-4. The first conductive via 243-1 carries the V_(n) supplyvoltage for both Wheatstone bridge circuits. The second conductive via243-2 is connected to the S_(n) output node of the first die as well asthe S_(P) output node of the second die. The third conductive via 243-3is connected to the V_(P) input voltage for both Wheatstone bridgecircuits. The fourth conductive via 243-4 carries the S_(P) output nodeof the first silicon die and the S_(n) output node of the second silicondie. Reference numeral 209 identifies the sidewall of the hole 208formed through the spacer 202.

FIG. 6E shows the layout of the second or lower oxide layer 220, whichis located between the second side 206 of the hollow spacer 202 and thebottom or second die 224. In FIG. 6E, reference numeral 209 shows wherethe sidewall of the hole 208 through the spacer 202 is located.

Finally, FIG. 6F shows the layout of the top or first side 226 of thesecond silicon die 224. P+ interconnects 252 connect the metal bond padsV_(p) 245-3 and V_(n) 245-1 to the piezoresistors of the secondWheatstone bridge 234 as shown. Other P+ interconnects 252 connect theoutput nodes of the Wheatstone bridge circuit 234 to bond pads 245-2 and245-4 and making electrical contact with conductive vias 243-2 and243-4. Reference numeral 209 identifies the location of the sidewalls ofthe hole 208 formed into the spacer 202.

A comparison of FIGS. 6B and 6F shows that S_(p) node of the Wheatstonebridge circuit 216 on the first die 210 is electrically connected to theS_(n) node of the Wheatstone bridge circuit 234 on the second die 224.Similarly, the S_(n) node of the Wheatstone bridge circuit 216 on thefirst die 210 is electrically connected to the S_(p) node of theWheatstone bridge circuit 234 on the second die 224. By cross-connectingthe S_(p) and S_(n) nodes of the two Wheatstone bridge circuits withinthe sensor structure, the number of bond pads required to makeconnections to the sensor can be reduced from six to four.

FIG. 7 is a cross-sectional view of a complete sensor 700. The completesensor 700 is comprised of a pressure sensing element 200 as describedabove, mounted in a plastic housing 702. The housing 702 is comprised ofa side wall 704 that surrounds a pocket 706, which is optionally filledwith a protective gel 720. The floor or bottom 718 of the pocket 706supports the pressure sensing element 200 on small dollops 710 ofadhesive that provide a seal around the opening 230 in the bottom 228 ofthe second die 224. A pressure port 712 is formed through the base 714of the housing 702, which permits liquid or gaseous fluids to applypressure to the second silicon die 224.

An application specific integrated circuit (ASIC) 716 is adhesivelybonded to the floor or bottom 718 of the pocket 706. The gel, 720, ifused, protects the pressure sensing element 200, the ASIC 716, and bondwires 724 that extend from the bond pads 244 of the pressure sensingelement to the bond pads (not shown) of the ASIC 716. Bond wiresconnecting the ASIC 716 to leadframes 708 are also protected by the gel720.

FIG. 8 is a cross-sectional view of an alternate embodiment of apressure sensor 800, which uses “flip-chip” assembly techniques. Apressure sensing element 200 as described above sits within a housing802 having an application specific integrated circuit (ASIC) 804, whichprovides signals to, and reads signals from the pressure sensing element200. The pressure sensing element 200 is flip-chipped or upside downwith a thicker substrate in the first silicon die 210 and a thinnersubstrate in the second silicon die 224. The first die 210 is formed tohave a channel or tube protrusion 225 that extends downwardly from thesecond side of the first silicon die 210 and which fits inside a squareor an annular groove 803 formed into the bottom of the housing 802. Thegroove 803 is partially filled with an adhesive 805, which holds theprotrusion 225 in the groove 803.

Conductive lead frames 806 extend between ball grid arrays (BGA) orelectrically conductive adhesive (ECA) 808 that attach both the ASIC 804and the pressure sensing element 200 to the lead frames 806. Bond pads816 located at the “bottom” of the pressure sensing element 200 areelectrically connected to the lead frames 806 using a BGA or ECA 808.Conductive vias 242 described above carry signals to the bond pads 816and BGA or ECA 808 from various layers of the pressure sensing element200. A lower pressure port 810 extends through the base 812 of thehousing 802 to allow liquids or fluids to exert pressure on thediaphragm 213 formed in the first silicon die 210.

One advantage of the pressure sensor depicted in FIG. 8 over thepressure sensor depicted in FIG. 7 is that in FIG. 8, gel is notoverlaid the pressure sensing element 200. Another advantage is thatwire bonding is not used. An optional under fill 814 surrounds theconnections provided by a ball grid array (BGA) 808. The under fill 814,is used, acts as an encapsulant that reduces oxidation of theconnections between the BGA 808 and the lead frames 806 and also helpsto hold the pressure sensing element 200 and ASIC 804 during vibrationor drop.

The foregoing description is for purposes of illustration only. The truescope of the invention is defined by the appurtenant claims.

What is claimed is:
 1. A pressure sensing element, comprising: a hollowspacer having first and second sides; a first silicon die attached tothe first side of the hollow spacer, the first silicon die having firstand second sides, the first side of the first silicon die including acircuit, electrical interconnects, a second insulated conductive viacoupled to the first insulated conductive via and a first silicondiaphragm; and a second silicon die attached to the second side of thehollow spacer, the second silicon die having first and second sides, thefirst side of the second silicon die including a circuit, electricalinterconnects and a second silicon diaphragm; whereby the hollow spacer,the first silicon die and the second silicon die form a cavity.
 2. Thepressure sensing element of claim 1, wherein the cavity is at leastpartially evacuated.
 3. The pressure sensing element of claim 1, whereinthe hollow spacer is silicon, and wherein the hollow spacer is fusionbonded to the first and second dies.
 4. The pressure sensing element ofclaim 1, wherein the hollow spacer is glass that is anodically bonded tothe first and second silicon dies.
 5. The pressure sensing element ofclaim 1, further comprising a first bonding layer attaching the firstside of the first silicon die to the first side of the hollow spacer. 6.The pressure sensing element of claim 1, further comprising a secondbonding layer attaching the first side of the second silicon die to thesecond side of the hollow spacer.
 7. The pressure sensing element ofclaim 5, wherein the first bonding layer is silicon oxide.
 8. Thepressure sensing element of claim 6, wherein the second bonding layer issilicon oxide.
 9. The pressure sensing element of claim 8, wherein thesecond bonding layer is a dielectric layer and configured to have ametal bond pad connecting a first end of the conductive via by a metalinterconnect through a contact window in the dielectric layer.
 10. Thepressure sensing element of claim 1, wherein the conductive via has afirst end and second end and the conductive via is comprised of at leastone of: metal; doped silicon.
 11. The pressure sensing element of claim1, wherein the first and second circuits are piezoresistive Wheatstonebridge circuits.
 12. The pressure sensing element of claim 11, whereinthe piezoresistive Wheatstone bridge circuits are comprised of fournodes, the first and second nodes of the first bridge circuit arecoupled to a first input voltage, the third node and fourth nodes of thefirst bridge circuit define a first absolute pressure output voltage,the first and second nodes of the second bridge circuit are coupled to asecond input voltage, the third and fourth nodes of the second bridgecircuit define a second absolute pressure output voltage, the differencebetween the first absolute pressure output voltage and the secondabsolute pressure output voltage being a voltage representative of thedifferential pressure between the first and second diaphragms.
 13. Thepressure sensing element of claim 11, wherein the piezoresistiveWheatstone bridge circuits are comprised of four nodes, the first andsecond nodes of the first circuit are coupled to corresponding first andsecond nodes of the second circuit, the third node of the first circuitis coupled to the fourth node of the second circuit at a first outputnode, and the fourth node of the first circuit is coupled to the thirdnode of the second circuit at a second output node, and wherein thealgebraic difference of signal levels at the first and second outputnodes is representative of a pressure difference between the first andsecond diaphragms.
 14. A pressure sensor comprised of: a housing havinga differential pressure sensing element and an integrated circuit (IC)coupled to the differential pressure sensor, the differential pressuresensing element being comprised of: a hollow spacer having first andsecond sides; a first silicon die attached to the first side of thehollow spacer, the first silicon die having first and second sides, thefirst side of the first silicon die including a circuit, electricalinterconnects, a second insulated conductive via coupled to the firstinsulated conductive via and a first silicon diaphragm; and a secondsilicon die attached to the second side of the hollow spacer, the secondsilicon die having first and second sides, the first side of the secondsilicon die including a circuit, electrical interconnects and a secondsilicon diaphragm; whereby the hollow spacer, the first silicon die andthe second silicon die form a vacuum cavity.
 15. The differentialpressure sensor of claim 14, further comprising a gel covering thedifferential pressure sensor and the IC.
 16. The differential pressuresensor of claim 14, further comprising a plurality of bond wires,configured to connect the differential pressure sensor to the IC andconnect the IC to the conductive leadframes, the plurality of bond wiresbeing embedded in the gel.
 17. The differential pressure sensor of claim14, wherein the housing has at least one port, coupled to one of thefirst and second diaphragms.
 18. The differential pressure sensor ofclaim 14, wherein the housing has a plurality of conductive leadframesconfigured to carry electrical signals between the differential pressuresensing element and the IC, and wherein the conductive leadframes arecoupled to the differential pressure sensing element and the IC by ballgrid arrays (BGA).
 19. The differential pressure sensor of claim 14,wherein the housing has a plurality of conductive leadframes configuredto carry electrical signals between the differential pressure sensingelement and the IC, and wherein the conductive leadframes are coupled tothe differential pressure sensing element and the IC by an electricallyconductive adhesive (ECA).
 20. The differential pressure sensor of claim18, further comprising an underfill, substantially covering the BGAs.21. The pressure sensing element of claim 14, wherein the first andsecond circuits are piezoresistive Wheatstone bridge circuits comprisedof four nodes, the first and second nodes of the first bridge circuitare coupled to a first input voltage, the third node and fourth nodes ofthe first bridge circuit define a first absolute pressure outputvoltage, the first and second nodes of the second bridge circuit arecoupled to a second input voltage, the third and fourth nodes of thesecond bridge circuit define a second absolute pressure output voltage,the difference between the first absolute pressure output voltage andthe second absolute pressure output voltage being a voltagerepresentative of the differential pressure between the first and seconddiaphragms.
 22. The pressure sensing element of claim 14, wherein thefirst and second circuits are piezoresistive Wheatstone bridge circuits,comprised of four nodes, the first and second nodes of the first circuitare coupled to corresponding first and second nodes of the secondcircuit, the third node of the first circuit is coupled to the fourthnode of the second circuit at a first output node, and the fourth nodeof the first circuit is coupled to the third node of the second circuitat a second output node, and wherein the algebraic difference of signallevels at the first and second output nodes is representative of apressure difference between the first and second diaphragms.